Lactate dehydrogenase (LDH) has five isoforms composed of combinations of H (heart) and M (muscle) subunits. In acute myocardial infarction, the pattern LDH1 > LDH2 ('flipped LDH') is diagnostic. Which subunit composition represents LDH1 and why is it predominant in cardiac tissue?
- A LDH1 = M4 (four M subunits); high in cardiac tissue because it converts pyruvate to lactate under anaerobic conditions
- B LDH1 = H4 (four H subunits); cardiac tissue primarily oxidises pyruvate aerobically so LDH1 preferentially converts lactate to pyruvate rather than the reverse ✓
- C LDH1 = H2M2; represents the mixed isotype with balanced activity in all tissues
- D LDH1 = H3M1; prevalent in liver and cardiac tissue equally
Explanation
LDH1 (H4) is composed of four H (heart-type) subunits and predominates in cardiac muscle, red cells, and renal cortex — aerobic tissues that preferentially oxidise pyruvate. The H subunit-containing isoforms have low Km for lactate and high Km for pyruvate, favouring the pyruvate-producing direction (lactate → pyruvate), supporting aerobic metabolism. The M subunit-containing LDH5 (M4) predominates in skeletal muscle and liver, favouring pyruvate → lactate (anaerobic glycolysis). In acute MI, elevated LDH1 with LDH1/LDH2 > 1 (flip) appears 24–48 h post-infarction and persists for 14 days.
Reference: Harper's Illustrated Biochemistry, 32nd ed.
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